1. Field of the Invention
The invention concerns the technical field of reinforcement, materials adapted to the creation of composite parts. More specifically, the invention concerns a new intermediate material containing a unidirectional layer for fabricating composite parts, by subsequent injection or infusion of thermosetting resin, a fabrication process for composite parts from a stack of such a material, as well as the obtained composite parts.
2. Description of Related Art
The manufacture of composite parts or goods, that is, comprising, on one hand, one or more reinforcements or fibrous layers, and on the other hand, a primarily thermosetting (“resin”) matrix and that can include thermoplastics, can for instance, be achieved by a process called “direct” or “LCM” (from the English “Liquid Composite Moulding”). A direct process is defined by the fact that one or more fibrous reinforcements are implemented in a “dry” state (that is, without the final matrix), the resin or matrix being implemented separately, for instance by injection into the mold containing the fibrous reinforcements (“RTM” process, from the English Resin Transfer Moulding), by infusion through the thickness of the fibrous reinforcements (“LRI” process, from the English “Liquid Resin Infusion or “RFI” process, from the English “Resin Film Infusion”), or alternatively by manual coating/impregnation by roller or brush on each unit layer of fibrous reinforcement, applied successively on the mold.
For the RTM, LRI or RFI processes, it is generally first necessary to build a fibrous preform of the mold of the desired finished product, then to impregnate this preform with a resin. The resin is injected or infused by differential pressure at temperature, then once all the amount of necessary resin is contained in the preform, the assembly is brought to a higher temperature to complete the polymerization/reticulation cycle and thus harden it.
The composite parts used in the automobile, aviation, or naval industry, are particularly subject to very strict requirements, notably in terms of their mechanical properties. Indeed, the mechanical properties of the parts are mainly related to a parameter which is the fiber volume ratio (VFR).
In these sectors, a large number of preforms are fabricated based on a reinforcement material, primarily carbon fibers, notably of the unidirectional type. It is possible to theoretically calculate the maximum fiber volume ratio contained in a unidirectional layer by assuming two types of structure: hexagonal or square. Assuming respectively a hexagonal-type structure and a square-type structure, the maximum VFR obtained is respectively 90.7% and 78.5% (An Introduction to Composite Materials, D. Hull, T. W. Clyne, Second Edition, Cambridge Solid State Science Series, 1996). In reality however, it appears difficult to obtain fiber volume fractions greater than 70% for composite parts. In practice, it is commonly accepted by the person skilled in the art, that a fiber volume ratio (VFR) of about 60% is standard for implementing satisfactory composite parts, along with good reproducibility (S. T. Peters, “Introduction, composite basics and road map”, in Handbook of Composites, Chapman & Hall, 1998, p. 1-20 and particularly p. 8),
The resin that is subsequently associated, notably by injection or infusion, to the unidirectional reinforcement layers during the creation of the part, can be a thermosetting resin, of the epoxy type for instance. In order to allow a correct flow through a preform composed of a stack of different layers of carbon fibers, this resin is most often very fluid. The major drawback of this type of resin is its fragility after polymerizational reticulation, which results in poor impact resistance of the fabricated composite parts.
In order to solve this problem, the prior art documents proposed the association of the unidirectional layers of carbon fibers with a web of thermoplastic fibers. Solutions such as these are notably described in the patent applications or patents EP 1125728, U.S. Pat. No. 628,016, WO 2007/015706, WO 2006/121961 and U.S. Pat. No. 6,503,856. The addition of this web makes it possible to improve mechanical properties in the compression after impact (CAI) test commonly used to characterize the impact resistance of the structures.
Document US 2006/0154545 describes such a solution in the case of a unidirectional fabric but which given the characteristics of the material described, does not make it possible to obtain a satisfactory VFR.
Some details on these prior solutions for unidirectionals are provided below. Patent application EP 1125728 in the name of Toray Industries Inc. describes a reinforcement material that associates a layer of reinforcement fibers to a short-fiber nonwoven material. The nonwoven is laminated on at least one face of the reinforcement layer, so that the fibers composing the nonwoven pass through the reinforcement fibers (of carbon) of the layer and are thus integrated into the reinforcement fibers. The nonwoven consists of a mix of low-melting point fibers and high-melting point fibers. It is important to note that all the cited examples use a single nonwoven material associated on only one face of the layer of reinforcement fibers consisting of a fabric or of a unidirectional layer, leading to a non-symmetrical reinforcement material. Example 4 uses a layer of reinforcement fibers consisting of a unidirectional fabric of 300 g/m2. The thickness of the nonwoven being used is not indicated, but it is certainly rather high, given its surface density (8 g/m2) and its indicated 90% void ratio. The stack used is of the type [−45/0/+45/90]2s, that is, 7 interplies containing a single nonwoven material. If the instructions in this document are applied to a layer of carbon fibers with a lesser surface density, 134 g/m2 for instance, the association with a same type of web, but on each side to obtain a symmetric material, would lead to a very low fiber volume ratio, non-compatible with the creation of primary structures for the aeronautic industry.
Patent application WO 2007/015706 in the name of The Boeing Company describes a method for the fabrication of preforms combining a stitched assembly that alternates layers of carbon fibers and layers of nonwoven materials to increase the impact resistance of composite structures. The nonwovens are placed at each interply and not on each side of the carbon fiber layers. This patent application does not mention any range of surface density for the carbon layers, nor a range of thicknesses for the nonwoven materials. The examples mention the use of three different nonwovens for which only the surface densities of 4.25 g/m2 (0.125 oz/yd2 in American units), 8.5 g/m2 (0.25 oz/yd2), and 12.7 g/m2 (0.375 oz/yd2) are specified. No indication is provided for the thickness of these products. One of the webs based on a copolyester actually has a negative effect on the impact resistance properties. The examples indicate the thickness of the created panels, the surface density of the carbon layers (190 g/m2) and the type of carbon fibers T700 with a volume density of 1780 kg/m3). The thicknesses vary from 0.177 to 0.187 inch (4.5 to 4.75 mm) for the panels with the best rupture stress results in compression after impact (CAI). From these thicknesses and the information about the type of fibers and the surface density of the carbon plies, it is possible to evaluate the VFR of the panels, which varies between 54 and 57%, lower than the value generally considered by the person skilled in the art for the fabrication of primary parts. The best CAI result (39.6 ksi or 273 MPa) is obtained for a VFR of 54%.
In patent application WO 2006/121961, a nonwoven material consisting of soluble fibers (of epoxy resins for instance) is intercalated at each interply of carbon fiber layers during the creation of the preform. The nonwoven is not directly associated with the carbon layer. The presented example uses a carbon fiber fabric with a surface density of 370 g/m2 with a nonwoven material of 60 g/m2. The fabricated plate makes it possible to obtain a VFR of only 55%. At the same time, the lack of precision about the compression alter impact CAI) test (no specification of the impact energy) does not make it possible to deduce the mechanical performances of the indicated measured value.
U.S. Pat. No. 6,503,856 mentions the use of a carbon layer on which two adhesive layers in the form of webs are superimposed on at least one side of the carbon layer. This patent does not indicate the thicknesses of the adhesive layers (only the diameters of the fibers of the two layers) and the preferred surface density of the carbon ranges from 200 to 1000 g/m2. Sources of electricity (batteries, fuel cells) are the target application for this type of product, and the relevance of such a product is not highlighted.
Consequently, it appears that the addition of a web to the techniques of previous art is carried out most often to the detriment of other mechanical properties. Indeed, as mentioned earlier, the mechanical properties are primarily determined by the volume fiber ratio (VFR) and the techniques described in previous art do not notably make it possible to obtain composite parts that have a VFR of the order of 60%.